Valve metals, which are understood in particular to be tantalum and its alloys, along with other metals in group IVb (Ti, Zr, Hf), Vb (V, Nb, Ta) and VIb (Cr, Mo, W) of the periodic table and their alloys, are widely used in component manufacture. The use of niobium and tantalum in the production of capacitors should be mentioned in particular.
The production of niobium or tantalum capacitors conventionally starts with corresponding metal powders, which are first compressed and then sintered to produce a porous compact. This is anodised in a suitable electrolyte, during which process a dielectric oxide film forms on the sintered compact. The physical and chemical properties of the metal powders used have a decisive influence on the properties of the capacitor. Decisive characteristics are, for example, the specific surface area and the content of impurities.
Tantalum powder in a quality allowing its use in the production of capacitors is conventionally produced by sodium reduction of K2TaF7. K2TaF7 is placed in a retort and reduced with liquid sodium. This produces a highly porous agglomerate of primary particles. Controlling the particle size of both the agglomerate and the primary particle and the porosity is particularly important in this reaction. The particle size of the primary particle is proportional to the specific surface area and therefore proportional to the specific capacity of the capacitor that is subsequently manufactured therefrom. It is particularly decisive here for the particle size of each individual particle to be as uniform as possible, since for every forming voltage there is an optimum particle size for the primary particle which results in the maximum specific capacity. The particle shape, particle size and porosity of the agglomerate determines the subsequent processing characteristics such as flowability and impregnatability and the resulting electrical properties such as equivalent series resistance (ESR) and equivalent series inductance (ESL). It can be deduced from this that for every application characterised by a desired capacity level and application voltage and anode size, a particle with the optimum primary and agglomerate particle size produces the best results.
It is known from U.S. Pat. No. 5,442,978 that the particle fineness can be influenced by the reaction temperature, an excess of reducing agent and by the dilution ratio of K2TaF7 in the salt bath. U.S. Pat. No. 5,442,978 therefore proposes that in order to produce tantalum powder having a high specific surface area, highly diluted K2TaF7 should be produced by the stepwise addition of sodium, the addition taking place at high speed. During the course of this reaction, irregular concentration ratios occur, such that the particle size distribution of the resulting powder is very wide.
According to U.S. Pat. No. 4,684,399 it is advantageous to add the tantalum compound continuously or stepwise during the reaction. This measure ensures that the concentration remains uniform during the reduction process.
In DE 33 30 455 A1 a doping agent is added to the reaction with the aim of obtaining a finer particle size. This allows widespread control of the primary particle but not of the agglomerate particle, since by virtue of the batch process this produces a wide agglomerate particle size distribution typical of stirred-tank reactors. In industrial practice, this particle is therefore first agglomerated further by application of heat and then laboriously reduced to the desired particle size distribution by mechanical methods (grinding, fractional screening, sieving). CN 1443618 describes a process which likewise results in uniform tantalum powders which, because of the process conditions, are contaminated with magnesium in a concentration of >20 ppm, however. Elevated magnesium contamination levels can have a negative influence on the subsequent electrical properties of the powder, however, particularly on the residual current.